A slab fed as the material to be rolled to a hot roughing mill of a hot strip mill has conventionally been manufactured by slabbing a steel ingot. Since, in the slabbing process, the slab width has been determined with the finishing width of a steel strip in view, the amount of slab edging by a hot roughing mill (i.e., the difference between the width of the slab fed to the hot roughing mill and the finishing width of a steel strip) has been relatively small as from about 10 to about 20 mm.
In the meantime, the continuous casting process which has various advantages over the slabbing process has recently been industrialized and has become popular in many applications, and this has made it difficult to feed many kinds of slabs with different widths to a hot roughing mill. The reason is that, in the continuous casting process, it is impossible to alter the slab width unless the mold is replaced, and this mold replacement causes a serious decrease in the productivity of slabs by the continuous casting process. As a result, the amount of slab edging by a hot roughing mill has largely increased to a value of from about 50 to about 75 mm.
In order to manufacture a steel strip at a satisfactory width accuracy by a hot finishing mill under such circumstances, it is particularly important to control the width of a slab during hot rough-rolling thereof. Major factors causing the occurrence of variations in the slab width during hot rough-rolling of the slab include those based on the slab fed to the hot roughing mill, and those based on heating and hot rough-rolling of the slab. Factors based on the slab fed to the hot roughing mill include the variation in the thickness and the width of the slab, the variation in slab dimensions caused by the local scarfing of the slab, and the variation in deformation resistance caused by the variations in the chemical composition of the slab. Factors based on heating of the slab are, for example, skid marks and the variation in deformation resistance caused by the non-uniformity of heating temperature in the heating furnace. Factors based on hot rough-rolling of the slab include the broadening of the slab width during rolling by horizontal rolls of the hot roughing mill, and the local narrowing of the slab width at the top portion and the bottom portion of a slab caused by the metal flow during rolling by vertical rolls of the hot roughing mill.
With reference to these various causes mentioned above, the state of variations in the slab width in the course of hot rough-rolling of a slab are shown in FIG. 1. In FIG. 1, (1) is the top portion of the slab; (2) is the middle portion of the slab; and, (3) is the bottom portion of the slab. As shown in FIG. 1, during hot rough-rolling in general, a serious narrowing of width occurs at the top portion (1) and the bottom portion (2) of the slab, and a variation in the width is observed also at the middle portion (2) of the slab.
There is conventionally known a method for correcting variations in the slab width during hot rough-rolling which comprises controlling the slab width principally by adjusting the roll gap of the vertical rolls of a hot roughing mill in response to the variation in the slab width. The following methods and apparatus have been proposed:
(1) A method, disclosed in Japanese Patent Provisional Publication No. 90,560/75 dated July 19, 1975, which comprises:
detecting the width of a slab transferred to a hot roughing mill provided with vertical rolls by means of a slab width detector installed at the entry or at the exit of said hot roughing mill;
calculating the deviations of the values thus detected from the target slab width at the entry or at the exit of said hot roughing mill; and,
controlling the slab width by adjusting the roll gap of said vertical rolls in response to said deviations (hereinafter referred to as the "prior art (I)").
(2) An apparatus, disclosed in Japanese Patent Publication No. 34,029/77 dated Sept. 1, 1977, which comprises:
a slab width measuring device for measuring the width of a slab at the exit of vertical rolls of a hot roughing mill in accordance with signals from a rolling load detector of said vertical rolls and a roll gap detector of said vertical rolls;
a slab width calculating device for performing a predicting calculation of the slab width at the exit of horizontal rolls of the hot roughing mill in accordance with the amount of slab width broadening caused by said horizontal rolls previously calculated and the signal from said slab width measuring device;
a slab width setting device for calculating a new slab width setting value, which predicts the effects acting on the finishing width of a steel strip at the final roll stand of a hot finishing mill, with the use of the width correcting coefficient of the steel strip at the exit of the final roll stand of the hot finishing mill and the width correcting coefficient of the slab at the exit of the final roll stand of the hot roughing mill; and,
a roll gap correction calculating device for calculating a roll gap correction value for the vertical rolls on the basis of signals from said slab width calculating device and said slab width setting device (hereinafter referred to as the "prior art (II)").
However, both the prior arts (I) and (II) presented above, in which the slab width is controlled during hot rough-rolling by adjusting the roll gap of vertical rolls, have the following problems:
(a) Control of the slab width by vertical rolls increases the ratio of crop loss occurring in the slab;
(b) For the purpose of increasing the control accuracy of slab width, it is desirable to effect adjustment of the slab width by the vertical rolls in the downstream of the hot roughing mill train as far as possible. The slab thickness decreases, on the other hand, toward the downstream of the hot roughing mill train. Adjustment of the slab width by the vertical rolls in the downstream of the hot roughing mill train may therefore cause buckling of the slab under the effect of the vertical rolls; and,
(c) As compared with horizontal rolls, vertical rolls are poor in the accuracy of roll gap adjustment and the roll gap response characteristics because of their structure. The control accuracy of slab width by the vertical rolls is therefore lower than that by the horizontal rolls.